Pesticide metaldehyde. To prevent snails and slugs from biting the young parts and fruits of strawberries, wounds are susceptible to diseases. Use metaldehyde granules to prevent and treat them. Metaldehyde is a blue granule, and the blue substance on strawberries is a pesticide. Residues of metaldehyde. Strawberry is a plant of the genus Fragaria in the Rosaceae family. Its fruits are mostly red and edible. It is a very popular fruit.